path: root/doc/manual/builtins.xml

<section xmlns="http://docbook.org/ns/docbook"
              xmlns:xlink="http://www.w3.org/1999/xlink">

<title>Built-in functions</title>


<para>This section lists the functions and constants built into the
Nix expression evaluator.  (The built-in function
<function>derivation</function> is discussed above.)  Some built-ins,
such as <function>derivation</function>, are always in scope of every
Nix expression; you can just access them right away.  But to prevent
polluting the namespace too much, most built-ins are not in scope.
Instead, you can access them through the <varname>builtins</varname>
built-in value, which is an attribute set that contains all built-in
functions and values.  For instance, <function>derivation</function>
is also available as <function>builtins.derivation</function>.</para>


<variablelist >

  
  <varlistentry><term><function>abort</function> <replaceable>s</replaceable></term>

    <listitem><para>Abort Nix expression evaluation, print error
    message <replaceable>s</replaceable>.</para></listitem>

  </varlistentry>

  
  <varlistentry><term><function>builtins.add</function>
  <replaceable>e1</replaceable> <replaceable>e2</replaceable></term>

    <listitem><para>Return the sum of the integers
    <replaceable>e1</replaceable> and
    <replaceable>e2</replaceable>.</para></listitem>

  </varlistentry>

  
  <varlistentry><term><function>builtins.attrNames</function>
  <replaceable>attrs</replaceable></term>

    <listitem><para>Return the names of the attributes in the
    attribute set <replaceable>attrs</replaceable> in a sorted list.
    For instance, <literal>builtins.attrNames {y = 1; x =
    "foo";}</literal> evaluates to <literal>["x" "y"]</literal>.
    There is no built-in function <function>attrValues</function>, but
    you can easily define it yourself:

<programlisting>
attrValues = attrs: map (name: builtins.getAttr name attrs) (builtins.attrNames attrs);</programlisting>

    </para></listitem>

  </varlistentry>

  
  <varlistentry><term><function>baseNameOf</function> <replaceable>s</replaceable></term>

    <listitem><para>Return the <emphasis>base name</emphasis> of the
    string <replaceable>s</replaceable>, that is, everything following
    the final slash in the string.  This is similar to the GNU
    <command>basename</command> command.</para></listitem>

  </varlistentry>

  
  <varlistentry><term><varname>builtins</varname></term>

    <listitem><para>The attribute set <varname>builtins</varname>
    contains all the built-in functions and values.  You can use
    <varname>builtins</varname> to test for the availability of
    features in the Nix installation, e.g.,

<programlisting>
if builtins ? getEnv then builtins.getEnv "PATH" else ""</programlisting>

    This allows a Nix expression to fall back gracefully on older Nix
    installations that don’t have the desired built-in function.
    However, in that case you should not write

<programlisting>
if builtins ? getEnv then __getEnv "PATH" else ""</programlisting>

    This Nix expression will trigger an “undefined variable” error on
    older Nix versions since <function>__getEnv</function> doesn’t
    exist.  <literal>builtins.getEnv</literal>, on the other hand, is
    safe since <literal>builtins</literal> always exists and attribute
    selection is lazy, so it’s only performed if the test
    succeeds.</para></listitem>

  </varlistentry>

  
  <varlistentry
  xml:id='builtin-currentSystem'><term><varname>builtins.currentSystem</varname></term>

    <listitem><para>The built-in value <varname>currentSystem</varname>
    evaluates to the Nix platform identifier for the Nix installation
    on which the expression is being evaluated, such as
    <literal>"i686-linux"</literal> or
    <literal>"powerpc-darwin"</literal>.</para></listitem>

  </varlistentry>


  <!--
  <varlistentry><term><function>currentTime</function></term>

    <listitem><para>The built-in value <varname>currentTime</varname>
    returns the current system time in seconds since 00:00:00 1/1/1970
    UTC.  Due to the evaluation model of Nix expressions
    (<emphasis>maximal laziness</emphasis>), it always yields the same
    value within an execution of Nix.</para></listitem>

  </varlistentry>
  -->

  
  <!--
  <varlistentry><term><function>dependencyClosure</function></term>

    <listitem><para>TODO</para></listitem>

  </varlistentry>
  -->

  
  <varlistentry><term><function>derivation</function>
  <replaceable>attrs</replaceable></term>

    <listitem><para><function>derivation</function> is described in
    <xref linkend='ssec-derivation' />.</para></listitem>

  </varlistentry>


  <varlistentry><term><function>dirOf</function> <replaceable>s</replaceable></term>

    <listitem><para>Return the directory part of the string
    <replaceable>s</replaceable>, that is, everything before the final
    slash in the string.  This is similar to the GNU
    <command>dirname</command> command.</para></listitem>

  </varlistentry>

  
  <varlistentry><term><function>builtins.getAttr</function>
  <replaceable>s</replaceable> <replaceable>attrs</replaceable></term>

    <listitem><para><function>getAttr</function> returns the attribute
    named <replaceable>s</replaceable> from the attribute set
    <replaceable>attrs</replaceable>.  Evaluation aborts if the
    attribute doesn’t exist.  This is a dynamic version of the
    <literal>.</literal> operator, since <replaceable>s</replaceable>
    is an expression rather than an identifier.</para></listitem>

  </varlistentry>

  
  <varlistentry><term><function>builtins.getEnv</function>
  <replaceable>s</replaceable></term>

    <listitem><para><function>getEnv</function> returns the value of
    the environment variable <replaceable>s</replaceable>, or an empty
    string if the variable doesn’t exist.  This function should be
    used with care, as it can introduce all sorts of nasty environment
    dependencies in your Nix expression.</para>

    <para><function>getEnv</function> is used in Nix Packages to
    locate the file <filename>~/.nixpkgs/config.nix</filename>, which
    contains user-local settings for Nix Packages.  (That is, it does
    a <literal>getEnv "HOME"</literal> to locate the user’s home
    directory.)</para></listitem>

  </varlistentry>

  
  <varlistentry><term><function>builtins.hasAttr</function>
  <replaceable>s</replaceable> <replaceable>attrs</replaceable></term>

    <listitem><para><function>hasAttr</function> returns
    <literal>true</literal> if the attribute set
    <replaceable>attrs</replaceable> has an attribute named
    <replaceable>s</replaceable>, and <literal>false</literal>
    otherwise.  This is a dynamic version of the <literal>?</literal>
    operator, since <replaceable>s</replaceable> is an expression
    rather than an identifier.</para></listitem>

  </varlistentry>

  
  <varlistentry><term><function>builtins.head</function>
  <replaceable>list</replaceable></term>

    <listitem><para>Return the first element of a list; abort
    evaluation if the argument isn’t a list or is an empty list.  You
    can test whether a list is empty by comparing it with
    <literal>[]</literal>.</para></listitem>

  </varlistentry>

  
  <varlistentry><term><function>import</function>
  <replaceable>path</replaceable></term>

    <listitem><para>Load, parse and return the Nix expression in the
    file <replaceable>path</replaceable>.  Evaluation aborts if the
    file doesn’t exist or contains an incorrect Nix
    expression.  <function>import</function> implements Nix’s module
    system: you can put any Nix expression (such as an attribute set
    or a function) in a separate file, and use it from Nix expressions
    in other files.</para>

    <para>A Nix expression loaded by <function>import</function> must
    not contain any <emphasis>free variables</emphasis> (identifiers
    that are not defined in the Nix expression itself and are not
    built-in).  Therefore, it cannot refer to variables that are in
    scope at the call site.  For instance, if you have a calling
    expression
    
<programlisting>
rec {
  x = 123;
  y = import ./foo.nix;
}</programlisting>

    then the following <filename>foo.nix</filename> will give an
    error:

<programlisting>
x + 456</programlisting>

    since <varname>x</varname> is not in scope in
    <filename>foo.nix</filename>.  If you want <varname>x</varname>
    to be available in <filename>foo.nix</filename>, you should pass
    it as a function argument:

<programlisting>
rec {
  x = 123;
  y = import ./foo.nix x;
}</programlisting>

    and

<programlisting>
x: x + 456</programlisting>

    (The function argument doesn’t have to be called
    <varname>x</varname> in <filename>foo.nix</filename>; any name
    would work.)</para></listitem>

  </varlistentry>

  
  <varlistentry><term><function>builtins.isList</function>
  <replaceable>e</replaceable></term>

    <listitem><para>Return <literal>true</literal> if
    <replaceable>e</replaceable> evaluates to a list, and
    <literal>false</literal> otherwise.</para></listitem>

  </varlistentry>

  
  <varlistentry><term><function>isNull</function>
  <replaceable>e</replaceable></term>

    <listitem><para>Return <literal>true</literal> if
    <replaceable>e</replaceable> evaluates to <literal>null</literal>,
    and <literal>false</literal> otherwise.</para>

    <warning><para>This function is <emphasis>deprecated</emphasis>;
    just write <literal>e == null</literal> instead.</para></warning>
    
    </listitem>

  </varlistentry>

  
  <varlistentry><term><function>builtins.lessThan</function>
  <replaceable>e1</replaceable> <replaceable>e2</replaceable></term>

    <listitem><para>Return <literal>true</literal> if the integer
    <replaceable>e1</replaceable> is less than the integer
    <replaceable>e2</replaceable>, and <literal>false</literal>
    otherwise.  Evaluation aborts if either
    <replaceable>e1</replaceable> or <replaceable>e2</replaceable>
    does not evaluate to an integer.</para></listitem>

  </varlistentry>

  
  <varlistentry><term><function>map</function>
  <replaceable>f</replaceable> <replaceable>list</replaceable></term>

    <listitem><para>Apply the function <replaceable>f</replaceable> to
    each element in the list <replaceable>list</replaceable>.  For
    example,

<programlisting>
map (x: "foo" + x) ["bar" "bla" "abc"]</programlisting>

    evaluates to <literal>["foobar" "foobla"
    "fooabc"]</literal>.</para></listitem>
    
  </varlistentry>

  
  <varlistentry><term><function>builtins.pathExists</function>
  <replaceable>path</replaceable></term>

    <listitem><para>Return <literal>true</literal> if the path
    <replaceable>path</replaceable> exists, and
    <literal>false</literal> otherwise.  One application of this
    function is to conditionally include a Nix expression containing
    user configuration:

<programlisting>
let
  fileName = builtins.getEnv "CONFIG_FILE";
  config =
    if fileName != "" &amp;&amp; builtins.pathExists (builtins.toPath fileName)
    then import (builtins.toPath fileName)
    else { someSetting = false; }; <lineannotation># default configuration</lineannotation>
in config.someSetting</programlisting>

    (Note that <envar>CONFIG_FILE</envar> must be an absolute path for
    this to work.)</para></listitem>

  </varlistentry>


  <!--
  <varlistentry><term><function>relativise</function></term>

    <listitem><para>TODO</para></listitem>

  </varlistentry>
  -->

  
  <varlistentry><term><function>removeAttrs</function>
  <replaceable>attrs</replaceable> <replaceable>list</replaceable></term>

    <listitem><para>Remove the attributes listed in
    <replaceable>list</replaceable> from the attribute set
    <replaceable>attrs</replaceable>.  The attributes don’t have to
    exist in <replaceable>attrs</replaceable>. For instance,

<screen>
removeAttrs { x = 1; y = 2; z = 3; } ["a" "x" "z"]</screen>

    evaluates to <literal>{y = 2;}</literal>.</para></listitem>

  </varlistentry>

  
  <varlistentry><term><function>builtins.tail</function>
  <replaceable>list</replaceable></term>

    <listitem><para>Return the second to last elements of a list;
    abort evaluation if the argument isn’t a list or is an empty
    list.</para></listitem>

  </varlistentry>

  
  <varlistentry
  xml:id='builtin-toFile'><term><function>builtins.toFile</function>
  <replaceable>name</replaceable> <replaceable>s</replaceable></term>

    <listitem><para>Store the string <replaceable>s</replaceable> in a
    file in the Nix store and return its path.  The file has suffix
    <replaceable>name</replaceable>.  This file can be used as an
    input to derivations.  One application is to write builders
    “inline”.  For instance, the following Nix expression combines
    <xref linkend='ex-hello-nix' /> and <xref
    linkend='ex-hello-builder' /> into one file:

<programlisting>
{stdenv, fetchurl, perl}:

stdenv.mkDerivation {
  name = "hello-2.1.1";
  
  builder = builtins.toFile "builder.sh" "
    source $stdenv/setup

    PATH=$perl/bin:$PATH

    tar xvfz $src
    cd hello-*
    ./configure --prefix=$out
    make
    make install
  ";

  src = fetchurl {
    url = http://nix.cs.uu.nl/dist/tarballs/hello-2.1.1.tar.gz;
    md5 = "70c9ccf9fac07f762c24f2df2290784d";
  };
  inherit perl;
}</programlisting>
  
    </para>

    <para>It is even possible for one file to refer to another, e.g.,

<programlisting>
  builder = let
    configFile = builtins.toFile "foo.conf" "
      # This is some dummy configuration file.
      <replaceable>...</replaceable>
    ";
  in builtins.toFile "builder.sh" "
    source $stdenv/setup
    <replaceable>...</replaceable>
    cp ${configFile} $out/etc/foo.conf
  ";</programlisting>

    Note that <literal>${configFile}</literal> is an antiquotation
    (see <xref linkend='ssec-values' />), so the result of the
    expression <literal>configFile</literal> (i.e., a path like
    <filename>/nix/store/m7p7jfny445k...-foo.conf</filename>) will be
    spliced into the resulting string.</para>

    <para>It is however <emphasis>not</emphasis> allowed to have files
    mutually referring to each other, like so:

<programlisting>
let
  foo = builtins.toFile "foo" "...${bar}...";
  bar = builtins.toFile "bar" "...${foo}...";
in foo</programlisting>

    This is not allowed because it would cause a cyclic dependency in
    the computation of the cryptographic hashes for
    <varname>foo</varname> and <varname>bar</varname>.</para></listitem>

  </varlistentry>

  
  <varlistentry><term><function>builtins.toPath</function> <replaceable>s</replaceable></term>

    <listitem><para>Convert the string value
    <replaceable>s</replaceable> into a path value.  The string
    <replaceable>s</replaceable> must represent an absolute path
    (i.e., must start with <literal>/</literal>).  The path need not
    exist.  The resulting path is canonicalised, e.g.,
    <literal>builtins.toPath "//foo/xyzzy/../bar/"</literal> returns
    <literal>/foo/bar</literal>.</para></listitem>

  </varlistentry>

  
  <varlistentry><term><function>toString</function> <replaceable>e</replaceable></term>

    <listitem><para>Convert the expression
    <replaceable>e</replaceable> to a string.
    <replaceable>e</replaceable> can be a string (in which case
    <function>toString</function> is a no-op) or a path (e.g.,
    <literal>toString /foo/bar</literal> yields
    <literal>"/foo/bar"</literal>.</para></listitem>

  </varlistentry>

  
  <varlistentry xml:id='builtin-toXML'><term><function>builtins.toXML</function> <replaceable>e</replaceable></term>

    <listitem><para>Return a string containing an XML representation
    of <replaceable>e</replaceable>.  The main application for
    <function>toXML</function> is to communicate information with the
    builder in a more structured format than plain environment
    variables.</para>

    <!-- TODO: more formally describe the schema of the XML
    representation -->

    <para><xref linkend='ex-toxml' /> shows an example where this is
    the case.  The builder is supposed to generate the configuration
    file for a <link xlink:href='http://jetty.mortbay.org/'>Jetty
    servlet container</link>.  A servlet container contains a number
    of servlets (<filename>*.war</filename> files) each exported under
    a specific URI prefix.  So the servlet configuration is a list of
    attribute sets containing the <varname>path</varname> and
    <varname>war</varname> of the servlet (<xref
    linkend='ex-toxml-co-servlets' />).  This kind of information is
    difficult to communicate with the normal method of passing
    information through an environment variable, which just
    concatenates everything together into a string (which might just
    work in this case, but wouldn’t work if fields are optional or
    contain lists themselves).  Instead the Nix expression is
    converted to an XML representation with
    <function>toXML</function>, which is unambiguous and can easily be
    processed with the appropriate tools.  For instance, in the
    example an XSLT stylesheet (<xref linkend='ex-toxml-co-stylesheet'
    />) is applied to it (<xref linkend='ex-toxml-co-apply' />) to
    generate the XML configuration file for the Jetty server.  The XML
    representation produced from <xref linkend='ex-toxml-co-servlets'
    /> by <function>toXML</function> is shown in <xref
    linkend='ex-toxml-result' />.</para>

    <para>Note that <xref linkend='ex-toxml' /> uses the <function
    linkend='builtin-toFile'>toFile</function> built-in to write the
    builder and the stylesheet “inline” in the Nix expression.  The
    path of the stylesheet is spliced into the builder at
    <literal>xsltproc ${stylesheet}
    <replaceable>...</replaceable></literal>.</para>

    <example xml:id='ex-toxml'><title>Passing information to a builder
    using <function>toXML</function></title>
    
<programlisting><![CDATA[
{stdenv, fetchurl, libxslt, jira, uberwiki}:

stdenv.mkDerivation (rec {
  name = "web-server";

  buildInputs = [libxslt];
  
  builder = builtins.toFile "builder.sh" "
    source $stdenv/setup
    mkdir $out
    echo $servlets | xsltproc ${stylesheet} - > $out/server-conf.xml]]> <co xml:id='ex-toxml-co-apply' /> <![CDATA[
  ";

  stylesheet = builtins.toFile "stylesheet.xsl"]]> <co xml:id='ex-toxml-co-stylesheet' /> <![CDATA[
   "<?xml version='1.0' encoding='UTF-8'?>
    <xsl:stylesheet xmlns:xsl='http://www.w3.org/1999/XSL/Transform' version='1.0'>
      <xsl:template match='/'>
        <Configure>
          <xsl:for-each select='/expr/list/attrs'>
            <Call name='addWebApplication'>
              <Arg><xsl:value-of select=\"attr[@name = 'path']/string/@value\" /></Arg>
              <Arg><xsl:value-of select=\"attr[@name = 'war']/path/@value\" /></Arg>
            </Call>
          </xsl:for-each>
        </Configure>
      </xsl:template>
    </xsl:stylesheet>
  ";

  servlets = builtins.toXML []]> <co xml:id='ex-toxml-co-servlets' /> <![CDATA[
    { path = "/bugtracker"; war = jira + "/lib/atlassian-jira.war"; }
    { path = "/wiki"; war = uberwiki + "/uberwiki.war"; }
  ];
})]]></programlisting>

    </example>

    <example xml:id='ex-toxml-result'><title>XML representation produced by
    <function>toXML</function></title>
    
<programlisting><![CDATA[<?xml version='1.0' encoding='utf-8'?>
<expr>
  <list>
    <attrs>
      <attr name="path">
        <string value="/bugtracker" />
      </attr>
      <attr name="war">
        <path value="/nix/store/d1jh9pasa7k2...-jira/lib/atlassian-jira.war" />
      </attr>
    </attrs>
    <attrs>
      <attr name="path">
        <string value="/wiki" />
      </attr>
      <attr name="war">
        <path value="/nix/store/y6423b1yi4sx...-uberwiki/uberwiki.war" />
      </attr>
    </attrs>
  </list>
</expr>]]></programlisting>

    </example>

    </listitem>

  </varlistentry>

  
</variablelist>


</section>